We have isolated a mutant Syrian hamster melanoma cell line that exhibits the phenomenon of "bromodeoxyuridine (BUDR) dependence" These cells exhibit three unique properties: (1) They require high concentrations of BUDR for optimal growth, (2) they require high concentrations of BUDR for the continued expression of characteristics associated with transformed cells in vitro, and (3) they are able to grow with all of their thymidine residues in nuclear DNA replaced by BUDR. We will attempt to elucidate the mechanisms responsible for BUDR dependence, in order to obtain information on DNA related activities in mammalian cells and also on the control of expression of the transformed phenotype. The studies will involve quantitative determinations of BUDR incorporation into DNA, in vitro and in vivo assays of gene expression and malignancy, and analyses of membrane components. BIBLIOGRAPHIC REFERENCES: Bick, M.D. and Davidson, R.L. Nucleotide Analysis of DNA and RNA in Cells with Thymidine Totally Replaced by Bromodeoxyuridine. Somat. Cell Genet. 2:63-76, 1976